This invention relates to a digital signal transceiver comprising a transmitting part and a receiving part, the transmitting part receiving at its input a speech signal called original signal, and including:
source coder means including a plurality of source coders for compressing said speech signal and delivering a compressed signal at a given output rate, said compressed signal having a measurable distortion and
means for selecting a source coder from the plurality of coders.
The invention also relates to telephony equipment and a digital telecommunications system.
The invention likewise relates to a digital signal transmission process, comprising the following steps:
a receiving step of receiving a speech signal called original signal,
a source coding step of compressing said speech signal and delivering a compressed signal at a given output rate, said compressed signal having a measurable distortion, and
a selection step of selecting a source coder from a plurality of coders for realizing the source coding step.
The invention finally relates to a digital signal reception process, comprising a source-decoding step.
The invention is notably applied to any cellular terminal operating according to a digital telecommunications standard of the GSM type (Global System for Mobile communications), PCS1900 (Personal Communications System), PHS (Personal Handyphone System), TDMA (Time-Division Multiple Access), CDMA (Code-Division Multiple Access), WBCDMA (WideBand CDMA), UMTS (Universal Mobile Telecommunications System) etc.
Japanese abstract published under no. 08237711 A describes a transceiver of the type defined in the opening paragraph, for enhancing the quality of an audio signal to be transmitted. For this purpose, the transceiver comprises means for directly testing in the useful signal the performance of families of source coders successively selected from a list of available families and means for comparing each output signal with the original signal so as to transmit only the coded signal that comes nearest to the original signal.
The invention proposes a less expensive method for optimizing the compromise between transmission quality of the speech signal and capacity of the communication means in digital telecommunications devices that involve a source coding.
For this purpose, a device of the type defined in the opening paragraph is provided, characterized in that said selection means comprise:
calculation means for forming an estimate of the distortion of the compressed signal,
check means for comparing this estimate with set values and selecting a source coder in dependence on the result of this comparison.
Thus, each original signal undergoes only one source-coding test before the optimum coder is selected, while the same coder family is adhered to.
According to an important characteristic feature of the invention, the receiving part comprises a plurality of decoders compatible with said source coders and control means co-operating with said check means for automatically selecting from the plurality of decoders a decoder that is compatible with the source coder selected by said check means.
According to a particular embodiment of the invention, the source coder delivers an internal residual error signal and said calculation means use said error signal for estimating said distortion.
According to another embodiment, the calculation means comprise:
inverse source decoder means co-operating with the source coder means for producing a decoded signal based on said compressed signal,
means for comparing the decoded signal with the original signal to produce a residual error signal and
means for processing the residual error signal to derive said estimate therefrom.
A transmission process of the type defined in the opening paragraph is provided, characterized in that said selection step comprises:
a calculation sub-step for forming an estimate of the distortion of the compressed signal,
a check sub-step for comparing this estimate with set values and selecting a source coder as a function of the result of said comparison.
The invention finally provides a reception process of the type defined in the opening paragraph for decoding a compressed signal via a transmission process of the above type, characterized in that the reception process comprises a step of automatically selecting a decoder from a plurality of available decoders as a function of said selected source coder.